In my copending U.S. Pat. application Ser. No. 943,447, filed Sept. 18, 1978, now U.S. Pat. No. 4,218,922 I described two methods for detecting flaws or unacceptable conditions in solder joints wherein an acoustic signal, with or without an electrical current, was applied on one side of a solder joint, passed through the joint, and received on the other side. The input was modulated by the joint and a comparison between the modulated output and the input was used to identify bad solder joints.
It occurs many times that a discrete element or integrated circuit is soldered by its lead wires to a conductor pad on a printed circuit board wherein the "other side" of the solder joint is inaccessible. For example, this is the case where the lead is soldered to a conductor pad which passes through the substrate to connect elements on the other side. Therefore, it would be beneficial to have a non-destructive testing method whereby the test could be run on the solder pad itself. Common ultrasonic flaw detectors which apply an acoustic wave to the joint and receive back an echo from the flaw are good for some flaws but not for others which are more subtle.